1. Technical Field
The present invention relates to a hard disk drive (HDD) and, more particularly, to a seek-servo apparatus and method of a hard disk drive capable of moving a head to the correct location of a desired track.
2. Related Art
A hard disk drive includes a magnetic disc, a spindle motor, a transducer, a slider, a head gimbal assembly (HGA), an actuator arm, a voice coil, a magnetic assembly, a voice coil motor, and a bearing assembly. The transducer is positioned adjacent to the surface of the magnetic disc which is rotatably driven by the spindle motor so that the transducer can write or read information on the magnetic disc by magnetizing or sensing a magnetic field of the magnetic disc. The transducer may be realized as one transducer or as two transducers, including one for writing information and one for reading information. The slider, which can be integrated with the transducer, can be manufactured and employed to create an air bearing between the transducer and the surface of the magnetic disc. In addition, the slider can be integrated with the head gimbal assembly. The head gimbal assembly may be mounted on the actuator arm including the voice coil, and the voice coil may be located adjacent to the magnetic assembly in order to define the voice coil motor (VCM). When current is applied to the voice coil, torque is generated, and that makes the actuator arm rotate about the bearing assembly. When the actuator arm rotates, the transducer moves across the surface of the magnetic disc.
In most cases, information is stored in annular tracks of the magnetic disc, and in general, each of the tracks includes a plurality of sectors. Each of the sectors has a data field and an identification field. The identification field contains grey code information identifying a particular sector and track. In order to write/read information on/from another track, the transducer moves along the surface of the magnetic disc. The act of moving the transducer to access another track is referred to as a seek routine. In other words, during a seek routine, the voice coil motor is excited with current, and thus the transducer can be moved from a current track on the surface of the magnetic disc to a new track. A controller (not shown) for controlling the movement of the transducer and the actuator arm moves the transducer from the current track to the new track in accordance with a seek routine and a servo control routine. The servo control routine ensures that the transducer moves to a correct track location.
If a conventional seek-servo apparatus for performing a seek-servo routine in a hard disk drive (which commonly designates both a seek routine and a servo control routine) is ideal, in other words, if the apparatus does not have any delay time, the acceleration command [u(t)] of the head can be expressed by the following equation:u(t)=ÿw(t)+Kv[{dot over (y)}w(t)−{dot over (y)}(t)]+Kp[yw(t)−y(t)]  (1)where ÿw(t) represents a desired acceleration trajectory (hereinafter, referred to as “target acceleration”) of the head to be accelerated, ÿw(t) represents a velocity trajectory (hereinafter, referred to as “target velocity”) of the head, yw(t) represents a desired position trajectory of the head to be moved (hereinafter, referred to as “target position”), y(t) represents an actual distance by which the head actually moves over the magnetic disc (i.e., the actual position of the head), Kv and Kp represent a velocity constant and a position constant, respectively, C represents differentiation, and CC represents double differentiation. If the target acceleration of the head is sin(t), the target velocity is 1−cos(t), and the target position is t−sin(t).
The acceleration command of the head defined by Equation (1) is Laplace-converted. Then, transient response characteristics represented by a ratio between a Laplace-converted target position [Yw(s)/L{yw(t)}] and an actual position [Y(s)/L{y(t)}] can be expressed by the following equation:                                           Y            ⁡                          (              s              )                                                          Y              w                        ⁡                          (              s              )                                      =                                                            s                2                            +                                                K                  v                                ⁢                s                            +                              K                p                                                                    s                2                            +                                                K                  v                                ⁢                s                            +                              K                p                                              =          1                                    (        2        )            
Equation (2) shows that, if a time delay Td is 0, the actual position y(t) of the head correctly follows the target position yw(t) of the head. However, in order to move the head to a desired track location on the magnetic disc, the conventional seek-servo apparatus spends a predetermined amount of time Td to compute an acceleration command defined by Equation (1), and to vary the torque of the head using an actuator (not shown) in accordance with the computed acceleration command. If a time delay occurs for the predetermined time Td, an acceleration command u′(t) of the head can be expressed by the following equation:u′(t)=ÿw(t−Td)+Kv[{dot over (y)}w(t−Td)−{dot over (y)}(t−Td)]+Kp[yw(t−Td)−y(t−Td)]  (3)
Here, after Laplace-converting Equation (3), transient response characteristics represented by a ratio between a Laplace-converted target position [Yw(s)/L{yw(t)}] and a Laplace-converted actual position [Y(s)/L{y(t)}] can be defined by the following equation:                                           Y            ⁡                          (              s              )                                                          Y              w                        ⁡                          (              s              )                                      =                                                            (                                                      s                    2                                    +                                                            K                      v                                        ⁢                    s                                    +                                      K                    p                                                  )                            ⁢                              e                                                      -                                          T                      d                                                        ⁢                  s                                                                                    s                2                            +                                                K                  v                                ⁢                                  se                                                            -                                              T                        d                                                              ⁢                    s                                                              +                                                K                  p                                ⁢                                  e                                                            -                                              T                        d                                                              ⁢                    s                                                                                ≠          1                                    (        4        )            
Equation (4) shows that, if a time delay is not 0, the conventional seek-servo apparatus cannot make the actual position of the head correctly follow the target position. Finally, the conventional seek-servo apparatus causes an overshoot, introduced by the time delay Td, and thus cannot correctly move the head to a desired track location.